US presidential rivals George Bush and John Kerry have spent more than $600m on TV and radio ads in the run-up to tomorrow's election, making it the most expensive campaign in American political history.

The two candidates and their supporters have spent three times as much as in 2000 when Senator Al Gore narrowly lost the presidency to Mr Bush.

And the rise of multimillion pound campaigns from pressure groups such as the Swift Boat Veterans For Truth alongside the ad blitz by the Democrats and Republicans has taken the total spending on political and so-called issue advertising to over $1bn since the start of the year, according to US tracking group TNS Media Intelligence.

"The story this whole year has been the sheer volume of ads," said Evan Tracey, president of TNS Media Intelligence/Campaign Media Analysis Group. "This is an election that I think changes politics."

Though the result of the election is widely being described as too close to call, Mr Kerry's campaign is marginally ahead of his Republican rival.

The Democrats spent about $250m on TV and radio commercials between March and September, $10m more than Mr Bush's campaign spend.

And spending by left-leaning pressure groups such as The Media Fund stretches the Democrat spending lead still further.

The Media Fund, set up by a former aide to Bill Clinton, emerged as the highest spending liberal group, pumping $55m into political ads. Its main Conservative rival, Progress for America Voter Fund, spent just $20m.

While the 2004 presidential campaign has been one of the longest on record - the Republican party started its TV campaign against Senator Kerry in the spring - it has concentrated on around a dozen swing states where the outcome of Tuesday's election is expected to be decided.

Residents of Toledo, Ohio for example were blitzed with just under 15,000 TV and radio ads between March and September, according to TNS Media Intelligence research.

Tampa residents could have watched 200 ads in a single day on the penultimate Saturday before the election, while on the same day people living in other swing cities such as Las Vegas, Cleveland and Milwaukee had between 100-150 ads to contemplate.

But in around two-thirds of the country the volume of campaign ads has been much more subdued.

Most television spending has been on national and local cable channels, with the Democrats favouring the news channels and the Republicans targeting male voters and buying into sports and outdoor shows, TNS reports.

The Democrats have been the cannier media buyers, snapping up spots at lower prices even if this meant opting for slots showing at different times than they would have wanted.

Records from a TV station in Cleveland quoted by TNS, reveal that Kerry's campaign spent $250 for a single spot between 9 am and 10 am on October 5, compared with the $375 paid by the Republicans in the same period.